1. Field of the Invention
This invention relates to an electronic still camera (a still-video camera) having a mechanical diaphragm mechanism for controlling the exposure of an image surface.
2. Description of the Related Art
The image sensor which is used as the image pick-up device in an electronic still camera generally has a frame size of 6.6 .times.8.8 mm or 4.8.times.6.4 mm. In terms of the length of one side, this is 1/4 to 1/5 of the size (24.times.36 mm) of the silver chloride film used in ordinary cameras. For this reason, the focal length of the lens used also is proportional to the size of the image pick-up device and therefore is short, thus making it possible to realize a compact lens.
However, since the effective aperture of the diaphragm corresponding to the f-stop value also is small in proportion to the size of the image pick-up device, it is difficult to maintain a high precision with an iris diaphragm that relies upon the diaphragm blades used in a camera of the type that produces an image on silver chloride film. It is especially difficult to realize high precision with an iris diaphragm when the effective aperture is less than 1 mm.
Because high precision cannot be obtained with an iris diaphragm, correcting for an error in exposure requires a control operation which includes performing photometry again after the diaphragm is set and changing shutter speed based on the new photometric value so as to obtain the proper exposure.